The conventional railway car truck in use in North America for several decades has been the three-piece truck, comprising a pair of parallel side frames oriented longitudinally connected by a transversely mounted bolster. The bolster is supported on the side frames by spring sets. The wheel sets of the truck are received in bearing adapters placed in leading and trailing pedestal jaws in the side frame, so that axles of the wheel sets are parallel. The bearing adapters permit slight angular adjustment of the axles. The railway car is mounted on the center plate of the bolster, which allows the truck to pivot with respect to the car. The spring sets permit the side frames to move somewhat with respect to the bolster, about the longitudinal, vertical, and transverse axes.
On straight track, a three piece truck with parallel side frames and parallel axles perpendicular to the side frames (i.e., a perfectly “square” truck) rolls without inducing lateral forces between the wheel flange and the rail. However, at high speeds, minor perturbations in the track or in the equipment can lead to a condition known as “hunting,” which describes an oscillating lateral movement of the wheel sets that causes the railcar to move side-to-side on the track. Hunting may be dangerous when the oscillations attain a resonant frequency.
Curved track poses a different set of challenges for the standard three-piece truck. When a railway car truck encounters a turn, the distance traversed by the wheels on the outside of the curve is greater than the distance traversed by wheels on the inside of the curve, resulting in lateral and longitudinal forces between the wheel and the rail. These wheel forces cause the wheel set to turn in a direction opposing the turn. On trucks with insufficient rigidity this results in a condition variously known as “warping,” “parallelogramming” or “lozenging,” wherein the side frames remain parallel, but one side frame moves forward with respect to the other. The “lozenging” condition can cause increased wear on the track and equipment, increase rolling resistance, and if severe enough result in a derailment.
In order to minimize hunting and to provide the standard three-piece truck with the ability to negotiate turns, the truck is generally designed to allow a nonparallel condition of the axles during the turn, which is then recovered on straight track. This may be achieved by permitting relative movement of the bearing adapters within the pedestal jaws of the side frames.
In order to improve curving performance, it is known to interpose an elastomeric bearing member between the side frame and the tops of the bearing adapters. The elastomeric member permits the side frames to maintain a ninety degree relationship with the wheel sets on straight track, while on curved track allowing the wheel sets some freedom of movement to depart from a square relationship to respond to turning forces and accommodate the nonparallel condition of the axles. The elasticity of the member biases the truck to return to its square position. Various systems to securely attach elastomeric pads to the side frame pedestal jaw are described in the prior art, including U.S. Pat. No. 7,966,946 and U.S. Pat. No. 4,674,412, which also contains a description of the prior art related to elastomeric pads generally. However, the prior art disclosure relating to elastomeric pads fails to adequately provide a different spring rate in the longitudinal direction compared to the lateral direction.
The prior art is also replete with systems for maintaining the bearing adapter securely in place in the pedestal jaw. U.S. Pat. No. 5,503,084, for example, describes a truck having a system for holding the bearing adapter in position within the pedestal jaw using tie rods running through a bore in the bearing adapter to prevent the bearing adapters from rotationally moving.
U.S. Pat. Nos. 7,845,288; 7,739,961; and 7,497,169 describe interfaces between a side frame pedestal jaw and a bearing adapter that may include a shear pad, wear plate, as well as other elements.
U.S. Pat. No. 3,844,226 describes a system to dampen or reduce the lateral forces transmitted to the side frame causing the lateral oscillations phenomenon (“hunting”) wherein a non-metallic surface with a low friction coefficient is positioned between a pedestal jaw and a bearing adapter, allowing the wheel set to move side to side in each direction with respect to the truck side frame, without transmitting force to the large masses of the truck parts.
U.S. Patent Application Publication No. 2014/0060380, which is incorporated by reference in its entirety, discloses that an interconnection between a side frame and wheel set with a high spring constant in the longitudinal direction relative to the lateral direction is advantageous to truck steering and riding performance. In specific embodiments, a longitudinal spring constant of about 20,000 lb/in to about 40,000 lb/in, and a lateral spring constant in the range of about 3,000 lb/in to about 5,000 lb/in was found to yield improved results over the prior art. It is desirable to provide additional modes to accomplish these performance objectives.